1. Field of the Invention:
This invention relates to a superconducting composite wire that retains ductility and resistance to fracture after fabrication. More specifically, the invention relates to a conducting metal-sheathed composite wire with submicron-particle superconducting powder core or filaments and a method for making the same.
2. Description of the Prior Art:
The general concept of making a composite wire by filling a tube or sheath made from a ductile metal, sealing the ends and then fabricating a wire having an encapsulated core is well recognized and documented in the literature and has been employed historically for a variety of purposes. For example, in U.S. Pat. No. 2,888,740 a special end use welding rod is produced by such a method by filling a tube or sheath of a ductile metal before fabrication with a preselected amount of core composition that results in a hard non-ductile alloy melt post welding. Similarly, various alternative methods of fabrication have been previously suggested and employed with varying degrees of success. Thus, for example, U.S. Pat. No. 3,922,769 describes a particularly useful technique of hydrostatically extruding a composite billet into a wire at elevated temperatures and very high pressures.
Application of the general concept of making composite wire, wherein the wire is intended to be used as a superconductor, has been far more limited in that it has been directed towards a specific approach to solving specific problems. As is known, superconductivity of metallic materials, i.e., the disappearance of the electrical resistance in the range of low temperatures, particularly near absolute zero, is not limited to specific pure metals or to alloys of two superconductors. Alloys of a normal conducting material and a superconducting material and alloys of two normal conducting materials show superconductivity as well. Intermetallic compounds have gained particular importance as superconductors since their transition temperatures markedly exceed those of pure components and alloys. In U.S. Pat. No. 3,256,118 the problems associated with the manufacture of superconducting wires of intermetallic compounds is expressly described as being due to the brittleness of these compounds. In this patent a method of manufacturing a superconducting wire is disclosed wherein a tube of one component is densely filled with a similar wire bundle and voids are filled with a second reactant of the intermetallic compound before this ductile composite is fabricated into a wire. Thereafter the superconducting intermetallic compound is produced, in situ, by reaction annealing the wire for several hours at high temperatures. Unfortunately, the ductility associated with the reactant composite is destroyed during the reaction annealing and formation of the superconducting structure.
Similarly, in U.S. Pat. No. 3,496,622 a sheath made of one component and filled with a powder mixture of the same component and the other reactant of a superconductor. This assembly is first fabricated into a wire and then reaction annealed, to promote the reaction forming the superconducting compound, but again resulting in a non-ductile wire. In U.S. Pat. No. 3,954,572 a multiple step reaction/diffusion process is disclosed wherein a composite ductile wire made up of a core and a jacket is first fabricated into wire and then an alloy sheath is applied to the composite wire, followed by heat treating to induce diffusion and reaction alternately, producing a non-ductile superconducting wire.
In U.S. Pat. No. 4,050,147 a fundamentally distinct technical approach to fabricating a composite superconducting wire is suggested. An ultrafine powdered superconductor is uniformly mixed with an equal or greater volume of a similarly powdered conducting metal such that the discrete superconducting particles are not in contact with each other. The blend is then encapsulated in a normal conducting tube or sheath. After fabrication into a wire the composite retains some ductility since a major part of the core consists of a ductile metal. The observed superconducting properties of the wire are characteristic of the superconducting powder dispersed in the normal conducting matrix. In this case the superconductivity is mediated by the, so-called, proximity effect. Contrary to the assertions of this patent the proximity effect is weak when the concentration of ductile metal is high. Consequently, superconducting properties of the wire are also weak and of limited practical interest.